Electrophotographic printers and copiers typically use light to produce a latent image on a photosensitive member by charging certain areas. The latent image is then developed by exposing it to a toner comprising electrically charged toner particles, in the presence of an electrode whose voltage is intermediate between the voltage of the exposed and unexposed areas. Depending on the charge of the toner particles, the toner is only attracted to the exposed areas, or only attracted to the unexposed areas. The toner particles are supplied in a carrier liquid when liquid toner is used.
The developed toner image is typically transferred first to a heated intermediate transfer member, and then transferred from the intermediate transfer member to a printing medium. Although the image can be transferred directly from the photosensitive member to the printing medium, the properties of the photosensitive surface are generally not ideal for printing, and better image quality is obtained by using an intermediate transfer member, with a surface whose properties (particularly temperature) may be optimized for printing.
A desirable property for the intermediate transfer member, and for the toner, is that the toner does not adhere to the intermediate transfer member very well, but that it does adhere well to a range of final printing media, so that all of the toner is transferred to the printing medium. An undesired consequence of avoiding adhesion of the toner to the intermediate transfer member is that the toner often does not adhere very well to the printing medium, especially for non-porous printing media such as plastic sheets.
U.S. Pat. No. 5,361,089, to Bearss et al, the disclosure of which is incorporated herein by reference, describes an electrographic printer which does not use an intermediate transfer member, and which uses a special colorless adhesive toner, in addition to the regular colored toner used to develop the image. The image is first developed on the photosensitive member, using one or more colored toners. Once the complete image is developed on the photosensitive member, the entire surface of the photosensitive member is exposed to light, which can, however, only reach those parts of the surface which are not covered with colored toner. The surface is then developed using the colorless adhesive toner, which has toner particles with a charge opposite to the charge of the toner particles of the colored toner. As a result, the adhesive toner is only attracted to those parts of the surface which are already covered with colored toner. The image, with the adhesive toner coating it, is then printed on the printing medium, and the adhesive toner causes the colored toner to adhere better to the printing medium.
U.S. Pat. No. 6,496,676, to Caruthers et al, the disclosure of which is incorporated herein by reference, describes an electrographic printer which uses an intermediate transfer member. After the toner image is transferred to the intermediate transfer member, the entire surface of the intermediate transfer member, both the areas that are covered with toner and those that are not, is coated with a cohesion increasing solution, which helps the toner image adhere better to the printing medium. The cohesion increasing solution, like the toner, is designed not to stick to the bare surface of the intermediate transfer member when the image is printed. The intermediate transfer member, with the toner image, is also exposed to a corona discharge before printing, which modifies the properties of the toner so that it adheres better to the printing medium.
US patent application 2003/0063922, the disclosure of which is incorporated herein by reference, describes using a powder toner particle coated with additives that improve its adhesion to the printing medium.
It is known to take printing media which do not have good adhesive properties, and to coat them with a special primer, which improves their adhesive properties, before printing.
It is sometimes desirable, for example in printing lottery tickets or prepaid phone cards, to print an image in which a portion of the image does not adhere well to the printing medium, so that it can be scraped off easily, but the rest of the image does adhere well to the printing medium.